Get someone to detail the problems which are directly attributable to the dropping of the netbook.
Send a copy of the report directly to BA, with a demand for reimbursement for of the costs.
If they refuse, you can file a small claims court claim, via the following link:
https://www.moneyclaim.gov.uk/web/mcol/welcome
You can do it all online for minimal cost. However, you should be aware that you will need to have a response to the following points
1. Why you placed your netbook under the seat in front of you (I assume this is an exit row), when this is expressly forbidden for safety reasons. It is unlikely that the "I have got away with it in the past" will work, as the removal of the netbook was for safety reasons.
2. Had you placed the netbook in the overhead bin yourself, you could have ensured that it was adequately stowed, and so you may have contributory negligence.
3. BA expressly advises, in their announcements that luggage can shift in flight and that extra care is required on opening the luggage containers to ensure that luggage is not loose.
I must stress, I feel BA may have some liability here, but you need to think through how you would respond to these points before suing, as they are likely to come up.